You have seen it before: a shirtless male jogger with a strap around his chest.

It is a heart-rate monitor and, by all accounts, it looks odd. It is as if the jogger is committing some sports fashion faux pas since the strap is worn just below the nipples.

But bizarre or no, this gadget is gaining popularity among runners. It tracks the heart rate, measured in beats per minute, which enables runners to train within a fixed zone without going above or below prescribed levels.

Mr Daniel Plews, exercise physiologist from the Singapore Sports Council, says: 'Originally, competitive athletes used such monitors to prescribe exercise intensity for their training programmes. Now, these gadgets are used by many individuals who just exercise to stay healthy and fit.'

The devices come mainly in the form of a wristwatch and a transmitter that is strapped to the chest. The chest unit detects electrical signals from the heart and transmits it to the watch, which processes the information and displays the figures.

Other models measure pulses in other parts of the body, such as in the hands and even the fingers.

Experts say the monitors also function as a stopwatch, an energy expenditure calculator, and to set personalised training zones.

More advanced versions can be synchronised with your personal computer and even come with a Global Positioning System (GPS) that tracks your speed and distance.

A set is priced from about $100 to more than $1,000.

Manufacturers say that they have seen an increase in sales of their products.

A spokesman for market leader Polar, which calls its monitors 'training computers', says sales increased by 12 to 15 per cent from last year. It will be launching a new series next month.

Crystal Time, the sole distributor here for American watch brand Timex, says sales of its heart-rate monitor watches grew by 10 per cent from last year.

Another brand Beurer, which boasts a model in the form of a glove, has seen a 35 per cent jump in product sales since its launch last year, says its local distributor Zen'Yu Pacific.

Mr Yap Tai Ann, a coach at fitness outfit Activ360, a subsidiary of Zen'Yu, says the Beurer PM100 glove model is more convenient than most traditional heart-rate monitors as there is no need for a chest strap.

Priced at $449, it monitors heart rate by emitting a light source and measuring the reflected light from changes in the blood-flow beneath the skin.

One such monitor user is British finance manager Nigel Town, an avid runner who will be taking part in the 10km competitive category of the Safra Singapore Bay Run and Army Half Marathon 2008 at the end of this month.

He runs about four times a week, including two training sessions with the team at Activ360, which is the official training partner for this year's Safra event.

The 51-year-old, who is currently working in Singapore, says that using the monitor helps him to remain consistent in his training.

'At the end of the run, I download the statistics into my computer to review the session and compare it with previous sessions,' he explains.

Sports physician Jason Chia from Changi Sports Medicine Centre adds: 'For those with health restrictions that require them to exercise within a certain intensity, heart-rate monitoring is an added safety measure.'

So how does one go about choosing such a monitor?

Mr Yap says: 'Such devices are used by more advanced sportsmen who want to monitor and improve their performance.

'Normal runners will not usually use it. Those who do might just want to use it to ensure they do not over-strain themselves.'

Indeed, if you really have to get your hands - or chest - on one of these devices, fitness instructor Wang Kee Leong, 30, says: 'Get a monitor that is suited for your needs. There is no point in getting a high-end one that has many functions you don't need.'